Iran appears to have been the ghost at the banquet at the Obama-Medvedev summit here in Prague. The sanctions currently being negotiated at the UN Security Council occupied much of the two leaders' time together and seem to have contributed to the half-hour delay in signing the Start treaty.

In his comments after the signing ceremony, Dmitry Medvedev agreed that action had to be taken against Iran which had "regrettably not responded to a series of constructive proposals" on its nuclear programme. "We cannot turn a blind eye to this," he said.

But Medvedev said that in a "straightforward and frank" discussion with Barack Obama he had "outlined our limits on sanctions". He said repeatedly that they should be "smart" and targeted, and not "create a catastrophe for the Iranian people".

He did not elaborate, but the Russian deputy foreign minister, Sergei Ryabkov, who is travelling with Medvedev, gave the example of an embargo on supplies of refined oil products - petrol and diesel - to Iran, which Ryabkov would trigger a "huge shock for the whole society and the whole population".

A refined oil blockade was being talked about last year but has since appeared to drop off the agenda. Britain opposed it, in part because it would be almost impossible to enforce and boost the smuggling industry on Iran's borders. The real issue now is how far the sanctions go in targeting the Revolutionary Guard. On one hand the guards have real control over the nuclear programme. On the other, they control much of the economy, so if you really go after them, you inevitably affect a lot of ordinary Iranians.

Obama said: "My expectation is that we are going to be able to secure strong, tough sanctions on Iran this spring." European diplomats believe that it will probably take longer, as China wants even milder measures than the Russians. It depends on how long spring lasts.